This invention generally relates to wireless communication systems. In particular, the invention relates to transmission of data in such systems where adaptive modulation and coding (AMC) and hybrid automatic repeat request (H-ARQ) techniques are applied.
In wireless communication systems, such as the third generation partnership project (3GPP) time division duplex (TDD) or frequency division duplex (FDD) communication systems using code division multiple access (CDMA) or orthogonal frequency division multiplex (OFDM) systems, AMC is used to optimize the use of air resources.
The modulation and coding schemes (sets) used to transmit data are varied based on wireless channel conditions. To illustrate, a type of data encoding (such as turbo versus convolutional coding), coding rate, spreading factor for CDMA system, modulation type (such as quadrature phase shift keying, M-ary phase shift keying versus M-ary quadrature amplitude modulation), and/or a number of sub-carriers for an OFDM system may change. If channel characteristics improve, a lower data redundancy and/or “less robust” modulation and coding set is used to transfer data. As a result, for a given allocation of radio resources, more user data is transferred resulting in a higher effective data rate. Conversely, if channel characteristics degrade, a higher data redundancy and/or “more robust” modulation and coding set is used, transferring less user data. Using AMC, an optimization between air resource utilization and quality of service (QOS) can be better maintained.
Data in such systems is received for transfer over the air interface in transmission time intervals (TTIs). Data within a TTI transferred to a particular user equipment is referred to as a transport block set (TBS). For a particular allocation of air resources, a less robust modulation and coding set allows for larger TBS sizes and a more robust modulation and coding set only allows for smaller TBS sizes. As a result, the modulation and coding set for a given radio resource allocation dictates the maximum size of the TBS that can be supported in a given TTI.
In such systems, a hybrid automatic repeat (H-ARQ) request mechanism may be used to maintain QOS and improve radio resource efficiency. A system using H-ARQ is shown in FIG. 1. A transmitter 20 transmits a TBS over the air interface using a particular modulation and coding set. The TBS is received by a receiver 26. A H-ARQ decoder 30 decodes the received TBS. If the quality of the received data is unacceptable, an ARQ transmitter 28 requests a retransmission of the TBS. One approach to check the quality of the received TBS is a cyclic redundancy check (CRC). An ARQ receiver 22 receives the request and a retransmission of the TBS is made by the transmitter 20. To increase the probability of successful delivery, retransmissions may apply a more robust modulation and coding set. The H-ARQ decoder 30 combines, the received TBS versions. A requirement for combining is that combined TBS sizes are identical. If the resulting quality is still insufficient, another retransmission is requested. If the resulting quality is sufficient, such that the combined TBS passes the CRC check, the received TBS is released for further processing. The H-ARQ mechanism allows for data received with unacceptable quality to be retransmitted possibly at a more robust MCS to ensure successful delivery and maintain the desired QOS.
Another approach is to retransmit the TBS using the old modulation and coding set. However, if the channel conditions dictate that a more robust modulation and coding set be used or the initial transmission was severally corrupted, the combining of the retransmitted TBSs may never pass, resulting in a transmission failure.
In a system using both H-ARQ and AMC, a change in modulation and coding set may be determined necessary to achieve successful delivery of a requested TBS retransmission. In this situation, the maximum amount of physical data bits allowed within the TTI varies with the modulation and coding set.
Since only one TBS exists per TTI, the effective user data rate corresponds to the TBS size applied to each TTI. To achieve maximum data rates the largest TBS size is applied to the least robust modulation and coding set within the TTI. When wireless channel conditions require more a robust modulation and coding set for successful transmission, such a TBS size can not be supported within the TTI. Therefore, each time a more robust modulation and coding requirement is realized, all outstanding transmissions in H-ARQ processes with TBS sizes not supported by this MCS and have not been successfully acknowledged are discarded.
In current implementations, when a TBS can not be successfully transmitted utilizing AMC and H-ARQ mechanisms, recovery is handled by the radio link control (RLC) protocol (at layer two). Unlike a H-ARQ recovery of failed transmissions, the RLC en-or detection, data recovery and buffering of a TBS queued in the node-B results in increased transport channel block error rates and transmission latency, potentially resulting in a failure to meet QOS requirements.
Accordingly, it is desirable to have alternate approaches to recover unsuccessful transmissions in such systems to allow for support of high data rates when AMC and H-ARQ techniques are applied.